New approaches to study historical evolution of mortality

New approaches to study historical evolution of mortality (with implications forecasting) Leonid A. Gavrilov, Ph. D. Natalia S. Gavrilova, Ph. D. Center on Aging NORC and The University of Chicago, Illinois, USA

Using parametric models (mortality laws) for mortality projections

The Gompertz-Makeham Law Death rate is a sum of age-independent component (Makeham term) and age-dependent component (Gompertz function), which increases exponentially with age. μ(x) = A + R e αx risk of death A – Makeham term or background mortality R e αx – age-dependent mortality; x - age

How can the Gompertz. Makeham law be used? By studying the historical dynamics of the mortality components in this law: μ(x) = A + R e Makeham component αx Gompertz component

Historical Stability of the Gompertz Mortality Component Historical Changes in Mortality for 40 -year-old Swedish Males 1. Total mortality, μ 40 2. Background mortality (A) Age-dependent mortality (Reα 40) 3. Source: Gavrilov, Gavrilova, “The Biology of Life Span” 1991

Historical Stability of the Gompertz Mortality Component Historical Changes in Mortality for 40 -year-old Japanese Women 1. Total mortality, μ 40 2. Background mortality (A) Age-dependent mortality (Reα 40) 3. Source: Gavrilov, Gavrilova, “The Biology of Life Span” 1991

Historical Stability of the Gompertz Mortality Component Historical Changes in Mortality for 40 -year-old Finnish Women 1. Total mortality, μ 40 2. Background mortality (A) Age-dependent mortality (Reα 40) 3. Source: Gavrilov, Gavrilova, “The Biology of Life Span” 1991

Cartogram of Age-Dependent (Biological) Mortality Component at Age 40. Men

Cartogram of Age-Dependent (Biological) Mortality Component at Age 40. Women

Predicting Mortality Crossover Historical Changes in Mortality for 40 -year-old Women in Norway and Denmark 1. 2. 3. 4. Norway, total mortality Denmark, total mortality Norway, agedependent mortality Denmark, agedependent mortality Source: Gavrilov, Gavrilova, “The Biology of Life Span” 1991

Predicting Mortality Divergence Historical Changes in Mortality for 40 -year-old Men and Women in Italy 1. 2. 3. 4. Women, total mortality Men, total mortality Women, agedependent mortality Men, age-dependent mortality Source: Gavrilov, Gavrilova, “The Biology of Life Span” 1991

Changes in Mortality, 1900 -1960 Swedish females. Data source: Human Mortality Database

In the end of the 1970 s it looked like there is a limit to further increase of longevity

Increase of Longevity After the 1970 s

Changes in Mortality, 1925 -2007 Swedish Females. Data source: Human Mortality Database

Age-dependent mortality no longer was stable In 2005 Bongaarts suggested estimating parameters of the logistic formula for a number of years and extrapolating the values of three parameters (background mortality and two parameters of senescent mortality) to the future.

Shifting model of mortality projection Using data on mortality changes after the 1950 s Bongaarts found that slope parameter in Gompertz. Makeham formula is stable in history. He suggested to use this property in mortality projections and called this method shifting mortality approach.

The main limitation of parametric approach to mortality projections is a dependence on the particular formula, which makes this approach too rigid for responding to possible changes in mortality trends and fluctuations.

Non-parapetric approach to mortality projections

Lee-Carter method of mortality projections The Lee-Carter method is now one of the most widely used methods of mortality projections in demography and actuarial science (Lee and Miller 2001; Lee and Carter 1992). Its success is stemmed from the shifting model of mortality decline observed for industrialized countries during the last 3050 years.

Lee-Carter method is based on the following formula where a(x), b(x) and k(t) are parameters to be estimated. This model does not produce a unique solution and Lee and Carter suggested applying certain constraints Then empirically estimated values of k(t) are extrapolated in the future

Limitations of Lee-Carter method The Lee-Carter method relies on multiplicative model of mortality decline and may not work well under another scenario of mortality change. This method is related to the assumption that historical evolution of mortality at all age groups is driven by one factor only (parameter b).

Extension of the Gompertz-Makeham Model Through the Factor Analysis of Mortality Trends Mortality force (age, time) = = a 0(age) + a 1(age) x F 1(time) + a 2(age) x F 2(time)

Factor Analysis of Mortality Swedish Females Data source: Human Mortality Database

Preliminary Conclusions There was some evidence for ‘ biological’ mortality limits in the past, but these ‘limits’ proved to be responsive to the recent technological and medical progress. Thus, there is no convincing evidence for absolute ‘biological’ mortality limits now. Analogy for illustration and clarification: There was a limit to the speed of airplane flight in the past (‘sound’ barrier), but it was overcome by further technological progress. Similar observations seems to be applicable to current human mortality decline.

Implications Mortality trends before the 1950 s are useless or even misleading for current forecasts because all the “rules of the game” has been changed

Factor Analysis of Mortality Recent data for Swedish males Data source: Human Mortality Database

Factor Analysis of Mortality Recent data for Swedish females Data source: Human Mortality Database

Advantages of factor analysis of mortality First it is able to determine the number of factors affecting mortality changes over time. Second, this approach allows researchers to determine the time interval, in which underlying factors remain stable or undergo rapid changes.

Simple model of mortality projection Taking into account the shifting model of mortality change it is reasonable to conclude that mortality after 1980 can be modeled by the following log-linear model with similar slope for all adult age groups:

Mortality modeling after 1980 Data for Swedish males Data source: Human Mortality Database

Projection in the case of continuous mortality decline An example for Swedish females. Median life span increases from 86 years in 2005 to 102 years in 2105 Data Source: Human mortality database

Projected trends of adult life expectancy (at 25 years) in Sweden

Conclusions Use of factor analysis and simple assumptions about mortality changes over age and time allowed us to provide nontrivial but probably quite realistic mortality forecasts (at least for the nearest future).

Acknowledgments This study was made possible thanks to: generous support from the National Institute on Aging Stimulating working environment at the Center on Aging, NORC/University of Chicago

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